This invention relates to powder metallurgy, and in particular to improving the machinability of sintered powder metal components by injecting into the component a layer of grease which penetrates the surface of the component to a given depth.
In large quantity manufacture of components made by powder metallurgy (P/M), there is often a need to improve the shape or increase the precision of a dimension by localized machining operations. Steel products made by P/M are usually harder to machine than their traditional steel or cast iron equivalents. This is thought by many in this field to be due to the presence of interconnected microporosity that produces micro vibrations at the cutting tool edge as it jumps across the pores.
There are several well-known and published ways to improve the ease of machining (machinability) of powder metal parts, but the methods are generally expensive. Included in the well-known methods is the addition, to the P/M steel powder blend prior to compaction, of machinability aides also in powder form, such as manganese sulfide or calcium fluoride. Other well known methods include: filling, of the micro-pores of the P/M steel matrix with an anaerobic plastic resin, or fully impregnating the pores with oil containing machining enhancement additives. All of these options treat the whole body of the component, whereas in fact, only a shallow surface region actually needs the machining enhancement. Therefore, these methods are wasteful and costly.
An alternative approach is to change the chemical composition of the surface region to be machined, for example by reducing the P/M steel carbon content. This local decarburization involves additional thermal or chemical processing, which adds to the cost. In addition, it is well known that surface decarburization is harmful to P/M steel strength under cyclic fatigue stress conditions.
In a particular case, namely piston-combustion-engine main bearing caps, all of the above measures have been tried with varying degrees of success in order to improve the machinability of the P/M steel.
The main bearing caps are arch-shaped, each having a half-bore which is machined to accept a crankshaft bearing, shell, and functions to locate and retain the crankshaft in its position below the cylinder block of a combustion engine. The main bearing cap must withstand the full force of the engine power as the combustion power is transferred from linear to rotational motion via the crankshaft. To provide a very round bore so as to ensure a smooth and quiet running engine, it is necessary to carry out a machining operation called line-boring. This involves simultaneously boring precision holes in the assembly of the cylinder block and bearing caps in order to provide a seating for the bearing shells that in turn contain the crankshaft. The cylinder block is usually made from cast iron or from an aluminum alloy. Therefore the boring operation is cutting two semi-circles, one in the cylinder block and one in each main bearing cap, in dissimilar materials simultaneously (either aluminum alloy and P/M steel or cast iron and P/M steel). This is commonly known as bi-metallic machining. Since the machining parameters, which include cutting, speed, feed rate, and depth of cut, are usually optimized to a single material bi-metallic machining often demands a compromise in cutting conditions selected. Since the smaller and cheaper component of the two being machined is the P/M steel main bearing cap, this is usually the focus of attention regarding improvement of machinability.
Experimentation has determined that local surface impregnation of a P/M steel bearing cap with oil containing solid lubricant additives produces a significant improvement in cutting tool life. This improvement was evaluated by setting-up a simulation of an engine producer""s block-line. P/M steel bearing caps were bolted to either aluminum casings or cast iron castings, which represent the respective cylinder block. The number of standard xe2x80x9ccutsxe2x80x9d that could be achieved before the cutting tool failed (it either wore excessively or lost its cutting edge by chipping) was measured. It was found that local impregnation with oil increased tool life by 200% to 300% in these tests.
It was surprising, however, that this success was not repeated when the same components were sent to a car engine manufacturer to be machined on the block-line at their facility. An intense investigation finally gave an answer to this apparent anomaly. It was round that freshly oil-impregnated surfaces provided lubrication and cooling of the cutting insert during in-house tests, but, after a period of time in transit and in storage before machine testing at the engine block-line, the oil layer soaked away into the body of the bearing cap, leaving very little behind to aid machinability. The mechanism of soaking away is known as capillary attraction and is based upon well-known physical laws governing fluid surface tension and diameters of holes (micro-pores).
This discovery prompted a research and development program to find a localized impregnant that would not soak away, but would xe2x80x9cstay putxe2x80x9d. After trying many different approaches, it was concluded that if the oil can be made to impregnate the P/M steel micro-pores under conventional impregnating conditions (normally a vacuum), then it would not xe2x80x9cstay putxe2x80x9d over a potential transit and storable period of one to six months. It appears that the readiness to impregnate is mirrored in the tendency to migrate away from the surface layer.
A potential though costly solution is conventional full oil impregnation. A method to achieve this is to place the P/M steel components in a steel basket, which is lowered into and sealed into a vacuum chamber. A vacuum is drawn, which pulls the air out of the inter-connected micro-pores in the P/M steel leaving a vacuum behind. Oil is caused to be sucked into the chamber by the vacuum such that the parts become immersed. The vacuum is released and atmospheric air pressure enters the chamber and pushes the oil into the evacuated micro-pores thereby filling them. This is a slow process with a cycle time of 30 minutes to an hour, and since P/M bearing caps can weigh 2 to 4 lbs. each, productivity is slow and the process is costly. Also, since the entire part is filled with oil and there is about 14% of micro-porosity present by volume, the oil usage is substantial and therefore also costly.
A further drawback to using oil impregnant is the limited amount of solid lubricant additives in the form of special compounds that can be added to the oil. Such compounds as manganese sulfide, molybdenum disulfide, or elemental graphite are beneficial to tool life, but they limit the fluidity of the oil and also tend to segregate and settle-out during storage prior to processing. This requires constant oil agitation and even then there is a tendency to segregation leading to inconsistent composition. It would be ideal if more solid machining aid compounds could be added to the oil, held in suspension, and yet still impregnate the P/M steel under commercial vacuum conditions. This desirable combination was not possible to achieve in experimentation due to conflicting requirements.
The invention provides a sintered powder metal part which is prepared for having a surface machined by having a layer of grease injected into its surface. The grease does not leak or wick away from the surface and stays put to increase tool life during machining.
Preferably, the grease is injected to a depth which is at least equal to the depth of machining, to fully utilize the advantages of the invention.
In another aspect of the invention, the part may be formed with one or more ribs adjacent to the grease injected surface which have been compressed down into the parts. Prior to compressing or collapsing them into the part, the ribs act as seals during the injection process to keep grease from leaking out between the pressing tool clearances, away from the surface to be treated.
The invention also provides a method and apparatus for preparing a part incorporating the above features.
These and other objects and advantages of the invention will be apparent from the detailed description and drawings.